This invention relates to steam turbine crossover conduit expansion joints and more particularly to a molten caustic detector which can be installed in the steam turbine crossover conduit expansion joint.
Current fossil fuel power plants have crossover conduit expansion joint designs that generally utilize two-ply bellows with various numbers and sizes of convolutions. In the presence of molten sodium hydroxide or certain sodium hydroxide salt mixtures brought into the turbine by improper boiler feedwater addictive control, caustic stress corrosion cracking can cause the bellows' inner ply to fail. Present practice for detecting expansion joint failure is to monitor the pressure between the two plies in the bellows with a change in the pressure indicating an inner ply leak. Unfortunately, however, by the time such pressure change indication occurs, the bellows must be replaced since outer ply failure is then imminent. More importantly, failure of the bellows' inner ply is an indication of the extent of caustic corrodent which has passed through the turbine and which can cause even more serious failures inside the turbine resulting in long and expensive forced outages for the turbine generator.
It has been found that the cracking mechanism of the expansion joint bellows results from the molten phase of sodium hydroxide collecting in convolutions along the horizontal bottom or "six o'clock" position of the bellows as installed. Thus, to detect the molten phase of sodium hyroxide the optimum detector position would be in the previously mentioned "six o'clock" region. Typical temperature conditions of 550 to 750.degree. F and approximately 200 psi pressure in the crossover conduit expansion joint favor sodium hydroxide or sodium hydroxide salt eutectics as the most common molten steam born contaminant. It has been found that under these temperature and pressure conditions the molten sodium hydroxide or sodium hydroxide salt eutectics are good electrical conductors with their conductivity rising with their temperature.
From past experience and operating problems encountered during such experience, early detection of the presence of such caustics is highly desirable to avoid damage not only to the crossover conduit expansion joint, but also to other critical, susceptible turbine elements. It is also desirable that caustic detection occur before the crossover conduit expansion joint has failed to an extent not easily reparable. A third objective of such a caustic detector would be to have the ability to monitor several areas of the turbine system and respond to caustic exposure thereto.